1. Field of the Invention
The present invention relates directly to in-bore protection of deployable (e.g., folded) fins for full bore artillery projectiles.
2. Discussion of Related Art
Ballistically launched projectiles are often exposed to severe temperature and pressure conditions. Untreated aluminum fins show melting in gun chambers that can reach temperatures of 3000 degrees K, and pressures of 50,000 psi. Insulating the deployable fins, from such conditions is paramount to assuring their ability to function.
In the prior art, U.S. patents have been issued that describe protection for projectile fins. Examples are the Mudd patent U.S. Pat. No. 4,936,219 entitled xe2x80x9cFin Protection Devicexe2x80x9d and the Garner patent U.S. Pat. No. 5,474,256 entitled xe2x80x9cCombustible Fin Protection Devicexe2x80x9d. Garner describes a fin cover that is designed to burn completely off as it moves to the bore exit. Mudd offers a more mechanical system for fin protection that shields the fin until muzzle exit. Also, the Mudd patent is directed at fixed-fin configurations, as that is what is depicted in the above cited patent figures. There is no mention or teaching directed at how such an invention would be applied to a deployable fin configuration. Mudd""s general geometry description of a protective body consisting of a modified frustum is inapplicable to create protection for fins on a boattail, as shown in FIG. 1. Further, the Mudd patent does not show the use of trapped gun gas pressure within the protective body material to effectively disintegrate the protective body once the projectile exits the bore. The management of gun gas pressure to affect the discard or function of associated projectile parts (i.e., sabots, fins) is a terribly complex effort even to those skilled in the art, and as such is not obvious. The inconsistencies in managing high pressure gun gasses are compounded by factors such as: burn ignition delay, asymmetric propellant bed ignition, projectile gun barrel interactions, and material degradation (mainly water absorption). The Mudd patent does describe an internal pressure created by an explosive train to affect the opening of pivoted petals and eventual discard of the mechanism. But this has its own set of problems varying from adverse projectile tip-off rates to delayed discard and protective mechanism ignition train inconsistencies.
Accordingly, it is an object of the present invention to provide a protective cap to encase and conform to the rear section of artillery projectiles with deployable fins.
It is another object of the invention to provide a protective cap for deployable fins on artillery projectiles made from a material of sufficient porosity such that the cap disintegrates and discards on projectile muzzle exit.
It is a further object of the invention to provide a protective cap for deployable fins on artillery projectiles that integrates with the projectile obturator to form a temporary pressure in-bore seal.
It is another object of the invention to provide a method of retaining deployable fins on artillery in the stowed position before firing.
The foregoing and other objects are achieved by providing a low cost protective component that can be affordably implemented to offer protection of deployable fins for full bore artillery projectiles. The protective component interfaces with the projectile obturator to form a temporary seal against hot gun gasses. The composition and thickness of the protective component allows gas infiltration into the component material and essentially instant degradation upon shot exit.
The purpose of the insulating protective component is to keep hinged fins in the stowed position as well as insulate them from harmful gun gasses. The second part of the component""s function is to disintegrate/discard upon muzzle exit. This allows the fins to deploy. Upon shot exit the component discards and imparts no significant launch rates or mechanical impulses to the projectile.
The protective component""s function is determined by its geometry as well as material properties. It completely encases the rear of the projectile (i.e., everything back of the obturator). It also conforms to the shape of the projectile rear. One benefit to conforming to the shape of the projectile rear is that the it is supported by the projectile body and cannot crush under the chamber pressures as it is supported by the projectile body. Another benefit is that the additional chamber intrusion volume to accommodate it is minimal. Its front edge also interfaces with the obturator such that a seal can be formed. This seal creates a differential pressure that keeps the fins closed as long as the pressure difference exists. Its discard/disintegration on muzzle exit is aided by the choice of a material with medium porosity. This porosity allows seepage of high-pressure gun gas into its material. On projectile exit, the high-pressure gas tries to equilibrate and escape to the atmosphere and thus fragments/disintegrates it.